1. Field
The present disclosure relates to a catalytic system based on a rare earth salt that is particularly suitable for the cis-1,4 stereospecific polymerization of isoprene, and also to a process for synthesizing polyisoprene having a high cis-1,4 content using this catalytic system.
2. Description of Related Art
The synthesis of polyisoprene having a high content of cis-1,4 units on an industrial scale constitutes a major challenge for the tire industry since these are polymers which, modified or unmodified, have characteristics similar to those of natural rubber.
Chain polymerization via coordination catalysis constitutes the main chemistry for preparing synthetic polyisoprenes having a high content of cis-1,4 units. Use is generally made of a multicomponent catalytic system based on a rare earth metal consisting of at least:                an organic phosphoric acid salt or an organic carboxylic acid salt of one or more rare earth metals,        a hydrocarbon solvent, generally cyclohexane or methylcyclohexane,        an alkylating agent, generally consisting of a trialkylaluminium or a dialkylaluminium hydride, and, where appropriate,        a preforming conjugated diene monomer, essentially butadiene, and/or        a halogen donor.        
Such catalytic systems are described in particular in international patent applications WO 02/38635, WO 02/38636, WO 03/097708, WO 02/48218 A1, WO 2006133757 A1 and WO 2007045417 A1 in the name of the Applicant companies.
In order to reduce the production costs, it appears advantageous to carry out the polymerization of isoprene in the presence of a volatile aliphatic hydrocarbon solvent. This is understood to mean an aliphatic hydrocarbon solvent, the boiling point of which is below 40° C. at atmospheric pressure. Indeed, this would lead to an industrial process that is less expensive in terms of energy since the removal of the volatile solvent in the step of recovering the synthesized polymer is facilitated by its low boiling point.
In the generally continuous processes used in industrial units, the polymerization solvent is commonly recovered at the end of the synthesis in order to be used again upstream of the process. In this context, a major drawback of the catalytic systems most commonly used for synthesizing polyisoprene is that they are prepared and diluted in a hydrocarbon aliphatic solvent comprising at least 6 carbon atoms, such as cyclohexane or methylcyclohexane, as described in document WO 02/38635 or WO 02/48218 A1. The removal of such solvents, which will be denoted as “heavy” in the remainder of the document, requires significant extraction temperatures, much higher than the boiling point of the volatile polymerization solvent. Moreover, depending on the industrial process used and its configuration, the use of a catalytic system prepared in a heavy solvent would lead to a mixture of heavy solvent and of volatile solvent being obtained in the process, then generally to the accumulation of the heavy solvent introduced by the catalyst in the volatile polymerization solvent. This would lead to an expensive supplementary step of separating the volatile and heavy solvents in order to be able to continue to carry out the polymerization in the volatile solvent, according to the aforementioned advantage.